Chemical of the Week 3 – Carbonic Anhydrase

  
I am a fan of Twitter and the New Scientist tweets pop up now and again. Recently I found this one interesting and no it wasn’t because of the Take That lyrics it was that it is about a catalyst. Sometimes I love nothing better than trawling the Internet to find out as much info as possible about something chemically related. Now, I can record what I find in this blog which will be great for teaching as every year I always say at some stage ‘there was this thing I read about … I must find it for next class’ and spend wasted time re trawling !
Anyway back to carbonic anhydrase – this stood out for me as i figured it must be something to do with the carbonic acid- bicarbonate buffering system that is studied at A2 . 

  
These reactions are of great importance maintaining the acid -base balance in different tissues including the stomach and pancreas. This process is extremely important in all living animals. Carbonic anhydrase turns out to be the enzyme that catalyses the hydration of carbon dioxide and the dehydration of bicarbonate. So what actually is it – it’s a single polypeptide chain ( mw ~30,000) complexed to an atom of zinc and boy does this catalyst work – a million reactions per second. How does it work – now this is the interesting bit chemically. I’m going to try and explain in the most basic terms – a water molecule binds around the zinc coenzyme causing polarisation of the O-H bond weakening it. A histidine amino acid accepts a proton leaving a hydroxide ion ( ie water split up into ions) and the active site brings the hydroxide and carbon dioxide close enough to form bicarbonate. How clever is that little enzyme ! Now the good thing is that this biological workhorse has been recognised and there even was the 10th international carbonic anhydrase conference held in Holland this year. Also just read that modified carbonic anhydrase enzymes have been used in CCS, it just gets better ! Now go on I know you want to google CCS !

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